The present invention relates to a safeguard system for a speed increasing/reducing mechanical assembly in case of lubricating oil loss and can be applied to a helicopter main, intermediate or rear gear box, especially to a military helicopter.
In speed increasing/reducing mechanical assemblies, the lubricating oil performs two functions: reducing the friction at the gears and bearings and evacuating the power losses in the form of heat. In the case of a main gear box in a helicopter, exhausting the oil-carried heat unfurls usually under pressure inside an oil/air exchanger whereas, in the case of an "auxiliary gear box"- type low power box, oil transmits heat to the inner walls of the box casing by running and conduction and the outer wall of this casing transmits heat by open-air convection.
In case of oil loss in the lubricating system, the friction coefficients of the various mechanical parts increase considerably and, consequently, the portion of energy lost in the form of heat becomes important with respect to the mechanical power transmitted to the helicopter rotor. Moreover, thermal energy is practically not evacuated any longer, hence rapid temperature increase in the gears and bearings. Therefore, the materials undergo metallurgical transformations and their mechanical performances collapse, causing the loss of torque transmission consecutively to the destruction of a roller or a tooth or both these elements.
It is in case of oil loss in a gear box, and especially the main gear box, the helicopter is forced to land a few moments after the breakdown because of the rapid destruction of the internal supports, whereas shortly no more power will be transmitted to the rotor. This oil loss may have very serious consequences when flying over a hostile zone: sea, forest, urban train station, enemy territory, a helicopter forced then to land or alight on water in hazardous, let alone catastrophic, conditions.